Cleanup SwXMailMerge constructor.

Move all fixed initializers into constructor definition.

Change-Id: I7317deae43abf6a9e865c66969cb39a4b69de826
Reviewed-on: https://gerrit.libreoffice.org/7189
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>